A large part of today's traffic load in wireless communication comes from users inside physical structures such as office buildings, shopping malls, cafés and restaurants, etc. Providing these indoor users with good coverage, high bit-rate and spectrally efficient communication from outside base stations is very challenging due to, for example, the penetration loss that occurs when the communication signals propagates through the walls of the building.
A well-known solution for enhancing communication signals inside a building is to use outdoor-indoor repeaters. An outdoor-indoor repeater has a pick-up antenna placed on the outside of the building connected via a double-directional power amplifier to a donor antenna placed on the inside of the building. Thus, signals from the base station are received by the pick-up antenna on the outside of the building, where the signal level is high, and then the signals are re-radiated inside the building by the donor antenna, and thereby circumventing the penetration loss. In a similar manner, communication signals from user equipment (UE), such as mobile phones and portable computer equipment, inside the building are received by the donor antenna located inside the building, and re-radiated by the pick-up antenna on the outside of the building. Typically, the pick-up antenna of the repeater on the outside of the building is placed in line of sight (LOS) of the base station antenna to ensure good link quality.
A well-known way of improving spectral efficiency in a wireless communications system is to utilize a multiple-input multiple-output (MIMO) communication system. Provided that both transmitter and receiver uses more than one antenna (e.g. K transmit antennas and M receive antennas), and that the channels between different pairs of transmit and receive antennas have similar power and can be made orthogonal, gains in spectral efficiency on the order of min(M,K) are possible to achieve. To allow indoor users communicating via a repeater to fully benefit from MIMO gains there needs to be at least as many repeater antennas as there are antennas at the base station.
Outdoor-indoor MIMO repeaters ensure good indoor coverage which implies high signal strengths. However the indoor coverage is typically limited to the close vicinity of the repeater donor antennas which usually are co-located with the repeater itself. Furthermore, as mentioned above, there is usually LOS between the base station and the repeater pick-up antennas in order to ensure a good link quality for the desired communication signals, and it is well known in the art that it is hard to achieve MIMO channels of more than rank two in a LOS environment since the spatially separated antennas will experience high correlation. Achieving sufficiently low correlation in a LOS environment might require a very large spatial separation of the repeaters antenna elements, which might make it difficult to co-locate the indoor donor antennas with the repeater and the pick-up antennas. Having non co-located donor antennas could lead to severe power imbalances between the donor antennas when for instance communicating with user equipment closer to one of the donor antennas, which in its turn will result in lower MIMO gains.
Thus, finding a way to offer good indoor coverage with high bit-rate and spectrally efficient communication is therefore highly sought for.